1. Field of the Invention
The subject invention relates generally to directional couplers, and more particularly to variable couplers for use in microwave frequency bands.
2. Description of the Prior Art
Variable coaxial directional couplers have existed for many years. A typical unit is the HP-393A as set out in the 1969 Hewlett-Packard catalog. This coaxial variable attenuator uses a principle of a directional coupler to achieve a wide range of attenuation over a full octave. The 393A covers 5 to 120 dB from 500 to 1,000 MHz. With special high power terminations it will handle up to 200 watts average, and is useful for mixing signals while maintaining isolation. Its intended use, however, is entirely different from that of the invention. It was designed for laboratory test bench applications. Variable coupling is achieved by mechanically adjusting the physical separation of the auxiliary coupled transmission line from the main through line. Such a device could not readily be incorporated into a multi-port microwave distribution network.
U.S. Pat. No. 4,697,160, Clark, discloses a hybrid power combiner and amplitude controller, which either couples or not, in distinction from the subject invention which sets at one or more values and fine tunes with a capacitor or other susceptance. When a transmission line element is used as the variable susceptance the coupling is frequency dependent.
U.S. Pat. No. 4,433,313, Saint et al., discloses a microwave directional coupler for coupling between a waveguide and stripline in which capacitors and inductors are used to equalize the phase velocity in the two media, which is required for flat, constant coupling versus frequency and good directivity.